Nested Modeling of Watershed Precipitation

The U. S. Bureau of Reclamation's GCCRP (Global Climate Change Research Program) is concerned with the possible impacts of global change upon precipitation, environmental variables and streamflow in the Western United States. This paper presents an assessment of regional model simulations of the unusually heavy precipitation in the El Nino of 1983, and the dry 1988-89 winter season. It outlines the nested modeling approach of our GCCRP that will use synoptic-scale forcing from the global NCAR (National Center for Atmospheric Research) CCM (Community Climate Model) to initialize the regional Pennsylvania State University/NCAR Mesoscale Model version 4 (MM4) developed by Giorgi and Bates. Two phases of Reclamation's collaborative GCCRP research with NCAR are outlined. Phase 1 focuses on validation of the nested modeling approach in current climate. Phase 2 will apply the nested modeling to future climate to determine the long term impacts of global change. This paper focuses on results from phase 1.